1. Field of the Present Invention
The present invention relates to a motion measurement system, and more particularly to an integrated Global Positioning System (GPS)/Inertial Measurement Unit (IMU) micro system, which can produce highly accurate, digital angular rate, acceleration, position, velocity, and attitude measurements of a carrier under a variety of environment.
2. Description of Related Arts
In the past decade, an IMU or GPS receiver is commonly employed to determine the motion measurement of a carrier.
An IMU is a key part of an inertial navigation system (INS). Generally, an INS consists of an IMU, a microprocessor and associated embedded navigation software. The components of the IMU include the inertial sensors (angular rate producer and acceleration producer, traditionally called gyros and accelerometers or angular rate sensor and acceleration sensor) and the associated hardware and electronics. Based on the carrier acceleration and rotation rate measurements obtained from the onboard inertial sensors, the position, velocity, and attitude measurements of a carrier are obtained by numerically solving Newton""s equations of motion through the microprocessor.
In principle, an IMU relies on three orthogonally mounted inertial angular rate producers and three orthogonally mounted acceleration producers to produce three-axis angular rate and acceleration measurement signals. The three orthogonally mounted inertial angular rate producers and three orthogonally mounted acceleration producers with additional supporting mechanical structure and electronic devices are conventionally called an Inertial Measurement Unit (IMU). The conventional IMUs may be catalogued into Platform IMU and Strapdown IMU.
In the platform IMU, angular rate producers and acceleration producers are installed on a stabilized platform. Attitude measurements can be directly picked off from the platform structure. But attitude rate measurements can not be directly obtained from the platform. Moreover, highly accurate feedback control loops are required to implement the platform IMU.
Compared with the platform IMU, in a strapdown IMU, angular rate producers and acceleration producers are directly strapped down with the carrier and move with the carrier. The output signals of the strapdown angular rate producers and acceleration producers are angular rate and acceleration measurements expressed in the carrier body frame. The attitude measurements can be obtained by means of a series of computations.
A conventional IMU uses a variety of inertial angular rate producers and acceleration producers. Conventional inertial angular rate producers include iron spinning wheel gyros and optical gyros, such as Floated Integrating Gyros (FIG), Dynamically Tuned Gyros (DTG), Ring Laser Gyros (RLG), Fiber-Optic Gyros (FOG), Electrostatic Gyros (ESG), Josephson Junction Gyros (JJG), Hemisperical Resonating Gyros (HRG), etc. Conventional acceleration producers include Pulsed Integrating Pendulous Accelerometer (PIPA), Pendulous Integrating Gyro Accelerometer (PIGA), etc.
The inertial navigation system, which uses a platform IMU, in general, is catalogued as a gimbaled inertial navigation system. The inertial navigation system which uses a strapdown IMU is, in general, catalogued as a strapdown inertial navigation system. In a gimbaled inertial navigation system, the angular rate producer and acceleration producer are mounted on a gimbaled platform to isolate the sensors from the rotations of the carrier so that the measurements and navigation calculations can be performed in a stabilized navigation coordinated frame. Generally, the motion of the carrier can be expressed in several navigation frames of reference, such as earth centered inertial (ECI), earth-centered earth-fixed (ECEF), locally level with axes in the directions of north-east-down (NED) or east-north-up (ENU) or north-west-up (NWU), and locally level with a wander azimuth. In a strapdown inertial navigation system, the inertial sensors are rigidly mounted to the carrier body frame. In order to perform the navigation computation in the stabilized navigation frame, a coordinate frame transformation matrix is established and updated in a high rate to transform the acceleration measurements from the body frame to the navigation frame.
In general, the motion measurements from the gimbaled inertial navigation system are more accurate than the ones from the strapdown inertial navigation system. Moreover, the gimbaled inertial navigation system is easier to be calibrated than the strapdown inertial navigation system. But, a gimbaled inertial navigation system is more complex and expensive than a strapdown inertial navigation system. The strapdown inertial navigation systems become the predominant mechanization due to their low cost, reliability, and small size.
An inertial navigation system is based on the output of inertial angular rate producer and acceleration producer of an IMU to provide the position, velocity, and attitude information of a carrier through a deadreckoning method. Inertial navigation systems, in principle, permit self-contained operation and output continuous position, velocity, and attitude data of a carrier after loading the starting position and performing an initial alignment procedure.
In addition to the self-contained operation, other advantages of an inertial navigation system include the full navigation solution and wide bandwidth.
However, an inertial navigation system is expensive and is degraded with drift in output (position, velocity, and attitude) over an extended period of time. It means that the position errors, velocity errors, and attitude errors increase with time. This error propagation characteristic is primarily caused by many error sources, such as, gyro drift, accelerometer bias, misalignment, gravity disturbance, initial position and velocity errors, and scale factor errors.
Generally, the ways of improving accuracy of inertial navigation systems include employing highly accurate inertial sensors and aiding the inertial navigation system using an external sensor.
However, current highly accurate inertial sensors are very expensive with big size and heavy weight.
A GPS receiver has been commonly used to aid an inertial navigation system recently. The GPS is a satellite-based, worldwide all-weather radio positioning and timing system. The GPS system is originally designed to provide precise position, velocity, and timing information on a global common grid system to an unlimited number of adequately equipped users.
A specific GPS receiver is the key for a user to access the global positioning system. A conventional, single antenna GPS receiver supplies world-wide, highly accurate three dimensional position, velocity, and timing information, but not attitude information, by processing so-called pseudo range and range rate measurements from the code tracking loops and the carrier tracking loops in the GPS receiver, respectively. In a benign radio environment, the GPS signal propagation errors and GPS satellite errors, including selective availability, serve as the bounds for positioning errors. However, the GPS signals may be intentionally or unintentionally jammed or spoofed, and the GPS receiver antenna may be obscured during carrier attitude maneuvering, and the performance degrades when the signal-to-noise ratio of the GPS signal is low and the carrier is undergoing highly dynamic maneuvers.
As both the cost and size of high performance GPS receivers are reduced in the past decade, a multiple-antenna GPS receiver can provide both position and attitude solution of a carrier, using interferometric techniques. This technology utilizes measurements of GPS carrier phase differences on the multiple-antenna to obtain highly accurate relative position measurements. Then, the relative position measurements are converted to the attitude solution. The advantages of this approach are long-term stability of the attitude solution and relatively low cost. However, this attitude measurement system retains the characterization of low bandwidth and is susceptible to shading and jamming, and requires at least 3 antennas configurations for a three-axis attitude solution, and requires antenna separation enough for high attitude resolution.
Because of the inherent drawbacks of a stand-alone inertial navigation system and a stand-alone GPS receiver, a stand-alone inertial navigation system or a stand-alone GPS receiver can not meet mission requirements under some constraints, such as low cost, long-term high accuracy, high rate output, interrupt-free, etc.
Performance characteristics of the mutually compensating stand-alone GPS receiver and the stand-alone inertial navigation system suggest that, in many applications, an integrated GPS/IMU system, combining the best properties of both systems, will provide superior accurate continuous navigation capability. This navigation capability is unattainable in either one of the two systems alone.
The benefits offered by an integrated GPS/IMU system are outlined as follows:
(1) The aiding of the GPS receiver""s signal-tracking loop process with inertial data from the INS allows the effective bandwidth of the loops to be reduced, resulting in an improved tracking signal in a noisy and dynamic environment.
(2) An inertial navigation system not only provides navigation information when the GPS signal is lost temporarily, but also reduces the search time required to reacquire GPS signals.
(3) Inertial navigation system errors and inertial sensor errors can be calibrated while the GPS signal is available, so that the inertial navigation system can provide more accurate position information after the GPS signal is lost.
(4) The GPS enables and provides on-the-fly alignment of an inertial navigation system by means of maneuvering, eliminating the static initial self-alignment of the pre-mission requirements of the stand-alone inertial navigation system.
Conventional IMUs commonly have the following features:
High cost,
Large bulk (volume, mass, large weight),
High power consumption,
Limited lifetime, and Long turn-on time.
Conventional GPS devices systems can be catalogued into two families:
Full-functional GPS receivers, including display, I/O ports.
GPS OEM engine modules.
A conventional integrated GPS/IMU system also has the following features:
High cost,
Large bulk (volume, mass, large weight),
High power consumption,
Limited lifetime, and Long turn-on time.
These present deficiencies of conventional integrated GPS/IMU systems prohibit them from use in the emerging cost-sensitive commercial applications, such as control of phased array antennas for mobile communications, automotive navigation, and handheld equipment.
MEMS, or, as stated more simply, micromachines, are believed as the next logical step in the silicon revolution. It is forecasted that this coming step will be different, and more important than simply packing more transistors onto silicon. The hallmark of the next thirty years of the silicon revolution will be the incorporation of new types of functionality onto the chip structures, which will enable the chip to, not only think, but to sense, act, and communicate as well. MEMS inertial sensors offer tremendous cost, size, and reliability improvements for guidance, navigation, and control systems, compared with conventional inertial sensors.
Meanwhile, new horizons are opening up for GPS technology. A tiny, inexpensive GPS chip sets such as the Mitel GP2000 and the SiRFstar GRF1/LX Chip, which are small enough to fit into a cellular phone or hand-held computer but powerful enough to receive GPS satellite signals, are advanced now. Upcoming consumer electronics devices such as cellular phones are planed to use it.
Although the availability of MEMS angular rate sensors and MEMS accelerometers and GPS chipsets makes it possible to achieve an integrated GPS/IMU microsystem, it""s still challenging to design and fabricate a high accurate and reliable integrated GPS/IMU microsystem with features of low cost, small size, weight light, and low power consumption. Also, a high performance, small size, and low power consumption integrated GPS/IMU microsystem can create more commercial applications than a conventional GPS/IMU system.
A main objective of the present invention is to provide an integrated GPS/IMU method and micro system, which can produce highly accurate, position, velocity, attitude, and heading measurements of the carrier under dynamic environments.
Another objective of the present invention is to provide an integrated GPS/IMU method and microsystem, wherein successfully incorporates the MEMS inertial sensors and GPS chipset technologies.
Another objective of the present invention is that three axes of the Earth""s magnetic field vector measurement from a tiny Earth""s magnetic field detector, such as a magnetometer, is incorporated to stabilize the heading solution of the system of the present invention.
Another objective of the present invention is that the Kalman filter is implemented in real time to optimally blend the GPS raw data and the INS solution to obtain the blended navigation solution.
Another objective of the present invention is that a robust Kalman filter is implemented in real time to eliminate the possible instability of the integration solution.
Another objective of the present invention is that a temperature based scheduler, error estimator, and a current acting error estimator are co-operated to minimize the mismatching between the filter system modules and actual ones due to change of environment temperature, so that the system of the present invention can provide high performance and stable navigation solution over a wide range of environment temperature.
Another objective of the present invention is to use the inertial velocity and acceleration from a position and attitude processor, which are corrected by a Kalman filter, to aid the code and carrier phase tracking of the GPS satellite signals in a GPS receiver so as to enhance the performance of the integrated GPS/IMU in heavy jamming and high dynamic environments.
Another objective of the present invention is to improve the accuracy of the GPS receiver position and velocity solution by using a differential GPS method. To accurately determine the GPS receiver""s position and velocity at the centimeter level, the GPS carrier phase measurements are used and the differential GPS is employed.
Another objective of the present invention is that the self-contained INS extends the GPS solution as the GPS receiver loses lock on the GPS signals. Once the GPS receiver regains the signals and then estimates the receiver""s position and velocity, the output (position and velocity) of the GPS receiver is used to correct the drifted position and velocity of the INS.
Another objective of the present invention is that a data link is used to transfer the differential GPS correction data, such as position, velocity, and raw measurement corrections (pseudorange, range rate, and carrier phase corrections), from a GPS reference site (wherein a GPS receiver is established with a known position) to the micro integrated GPS/IMU system. Using the differential GPS and carrier phase measurements. the accuracy of the GPS positioning is of the order of centimeter level after fixing the integer ambiguities, and, as a result, the micro integrated GPS/IMU system is applicable to highly accurate position requirements.